1. Field of this Invention
The invention relates to an implantable prosthesis anchorage which consists of bioactive, resorbable ceramic materials on the basis of calcium phosphates and of polymers which are mechanically and chemically stable in the body. In addition, the invention relates to the production of the respective composite material, i.e., a system or a composite of materials for the cementless anchoring of highly stressed endoprostheses, especially joint endoprostheses or dental roots.
2. Prior Art
Anchoring of the predominantly metallic prostheses usual today is in general effected by means of a bone cement curing in situ, which mechanically anchors the shaft of the prosthesis in the "implant bed", i.e., an appropriate cavity made in the bone. This method is regarded as problematic today because the polymerising cement constituents release monomeric components to the surrounding tissue and thus initiates detrimental side reactions in the organism. In addition, the heat of reaction generated during polymerisation may cause destruction of the tissue. These and other reasons may lead to loosening of the prosthesis. The above disadvantages have given rise to various research activities aimed at developing cementless anchoring systems.
The improved tissue compatibility of highly stable oxide-ceramic prosthesis materials had suggested that these materials would be suitable for achieving direct bone-ceramic attachment (ingrowth of bone into the ceramic) and thus permit cementless achoring. Special surface structures of the prosthesis shaft characterised by grooves, cavities or the like were to foster the process of of mechanical anchoring. Unfortunately it has become obvious that this type of anchoring is not suitable for prostheses subject to high mechanical loads, e.g., the hip joint, where shear stresses occur at the implant bed; this is due to the fact that formation of a fibrous intermediate layer and atrophy of the bone finally lead to loosening of the prosthesis.
This finding has given rise to the use of so-called bioactive materials, which are to bring about direct contact between bone and materials surface without an intermediate fibrous boundary layer. These materials include, for example, calcium phosphates of specific compositions which ensure direct bone-prosthesis attachment without the formation of an intermediate fibrous boundary layer (cf. literature references: "Resorbierbare keramische Werkstoffe fur den Knochenersatz", Biomedizinische Technik, vol. 20, May 1975, p. 115 ff; "Neuere Werkstoffe in der medizinischen Technik," Chemie-Ingenieur-Technik, No. 8, 1975, pp. 327-333). In specific structural composition and phase structure, these calcium phosphates are degradable in a biological environment, i.e., they are resorbed by the cells participating in the bone transformation. Although this material property is desirable in specific cases, it nevertheless precludes the use of calcium phosphates as the sole material for permanently implanted prostheses.